In TreeModelNode there are places where UnknownTypeException is catched, but not
reported if the object is an instance of String. This does not seem to have any
sense, since this can hide implementation deficiencies in the models.

Due to compatibility reasons, it's risky to report all UnknownTypeExceptions.
Instead, we report them into the log, via a logger with Level.CONFIG level. This should be sufficient to catch eventual errors.